The present invention relates to a hydraulically operated continuously variable transmission having a hydraulic pump coupled to an input shaft, a hydraulic motor coupled to an output shaft, the hydraulic pump and the hydraulic motor being interconnected by a closed hydraulic circuit, and an auxiliary pump connectable to the closed hydraulic circuit and hydraulic actuator's for operating controlled members in the closed hydraulic circuit.
Hydraulically operated continuously variable transmissions are known in the art as disclosed in Japanese Laid-Open Patent Publication No. 56-52659, for example.
In such hydraulically operated continuously variable transmissions, control oil is supplied to hydraulic actuators and replenishing oil is supplied to a closed hydraulic circuit by a common auxiliary pump. The supply of control oil is given priority over the supply of replenishing oil by means of a circuit compensating valve. When the amount of oil discharged from the auxiliary pump is large, the discharged oil is returned to an oil tank by a relief valve.
In order to give priority to the supply of replenishing oil to the closed hydraulic circuit over the return of oil to the oil tank and also to stabilize the operation of the hydraulic actuators at all times, it is necessary to keep the pressure of control oil at a substantially constant level. According to the conventional design, a pressure level set for the circuit compensating valve is slightly lower than a pressure level set for the relief valve.
However, the process of establishing such pressure levels is difficult to perform. Moreover, since the difference between the pressure levels thus set is very small, the pressure levels may easily be reversed even by a slight amount of fatigue-induced permanent setting of springs in the valves during long/usage. When this happens, oil is returned to the oil tank earlier than it is supplied to the closed hydraulic circuit, which therefore fails to be supplied with a sufficient amount of oil.